The present invention relates to a safety switching device for actuating actuators in a fail-safe manner, and further relates to an emergency shut-off circuit comprising a safety switching device according to the present invention.
Safety switching devices and, in particular, safety relays are apparatuses intended to ensure the safety of humans working in the environment of an industrial process. Safety relays are for instance used to detect the opening of emergency stop switches or other machine lock-out switches, such as interlock switches guarding a gate or limit switches. Furthermore, safety relays are also used for processing the output signals of electro-sensitive protective equipment, such as light curtains or light grids.
Generally, all safety devices have to be designed to meet stringent requirements defined in worldwide adapted safety standards. These standards intend to achieve high reliability, which is achieved particularly by applying redundancy, diversity and monitoring principles. Safety relays, for example, provide internal checking or fault conditions, such as jammed, welded or stuck contacts of safety switches. Moreover, safety switches, such as limit switches, which already have redundant normally closed safety contacts for use with dual channel safety relays, are additionally provided with an auxiliary contact for status indication.
On the other hand, electro-sensitive protective equipment normally has so-called output signal switching devices, OSSDs, for generating an output signal to be connected to an input of the safety relay. These semiconductor outputs, which in the following will be referred as OSSDs, are provided as safety switching output of protective units, such as light grids or safety laser scanners. When the protective area is violated, the safety sensor switches the OSSDs into an OFF-state. Thus, the switching off of the machine or any endangering state is initiated. As this is generally known, each safety sensor has two parallel OSSD outputs, which are evaluated independently from each other in a two-channel modus. For instance, the terminal of an electro-sensitive protective equipment is connected to a safety relay or a safety controller according to category 3 of EN 954-1 (performance level d according to EN ISO 13849-1) via two OSSD outputs. The safety sensor transmits the status information “protective field free”, which will be evaluated by the safety control device or safety relay.
When using the conventional safety switching devices 200 as shown in FIGS. 6 and 8, it has to be determined by changing the settings at a configuration unit 201, whether semiconductor outputs (OSSD) or emergency shut-off circuits, such as protective doors or the like, are to be coupled with the safety inputs S12, S22 of the switching device 200. This is due to the fact that the semiconductor outputs of the OSSDs perform an inherent self-test regarding any short-circuits between the respective leads. With electro-sensitive protective equipment 110, consequently, a safety input of a safety relay 200 only has to be equipped for performing a self-test of its own hardware.
As shown in FIG. 6, a conventional safety device 200 has a configuration unit 201, for instance, comprising a switch 204, which selects a different operational mode depending on whether the safety device 200 is connected to a light curtain 110, having OSSD semiconductor outputs, or is used within an emergency shut-off circuit, as this is shown in FIG. 8. In contrast to the present invention, a cross fault monitoring is provided either by the light curtain 110, or the input terminals S11, S22, when the configuration is set for the emergency stop operation. The safety relay 200 in the application environment of FIG. 6 expects a static 24 V signal at the input terminals.
On the other hand, when connecting the safety switching device 200 with a safety shut-off circuit, the safety switching device has the task of monitoring the input conductors with respect to any possible cross-circuiting. Known emergency shut-off circuits, for instance, use clocking signals which are transmitted within the emergency shut-off circuit, as this is for instance shown in FIGS. 8 and 9. In this conventional arrangement, the terminals S11 and S21 output clocking signals of directly opposed polarity which are transmitted to the safety inputs S12, S22 in an unchanged pattern, if no fault condition has occurred. This signal pattern is recognized by the safety device as a safe state.
However, the safety switching device 200 according to FIGS. 6 and 8 must either have configuration means for choosing the settings in accordance with the field of application, or must have a plurality of different inputs, each configured for a different kind of application. Such configuration, however, is costly and also enhances the expenditure for installing a safety system.
The present invention therefore aims at overcoming the above-identified problems. In particular, an object underlying the present invention is to provide a safety switching device and an emergency shut-off circuit, comprising such a safety device, which can be used universally within different safety circuits without the necessity of setting a different configuration depending on the respective application field.